In projectors based on what is referred to as a “flying spot” principle of operation, light beams (typically consisting of the three primary colors red, green and blue) are deflected by means of a two-dimensional resonant micromirror and projected onto an image plane.
In a “flying spot” projection, light beams of different colors e.g. from laser sources (red R, blue B and green G) are in each case directed onto a semitransparent mirror (transmission and reflection of the mirrors are dependent on the wavelength) and then as a common beam (also referred to as a projection beam) onto a two-dimensional resonant micromirror which deflects the common beam two-dimensionally and projects it onto an image plane. In the process the image is built up in the image plane by means of the continuously harmonically deflected common beam.
Image information is generated and displayed synchronously with the deflection of the micromirror by means of intensity modulation of the respective light source.
As a result of the movement of the mirror, e.g. by means of line scan methods or Lissajous methods, and correspondingly suitable modulation of the laser intensity it is thus possible to generate the desired image information on the screen. The mirrors can be embodied e.g. as what are termed MEMS mirrors.
Image jitter effects (e.g. an image running through continuously horizontally or vertically) occur when the actual frequency of the mirror movement (row or column frequency) does not correspond to the reference frequency for the mirror movement set in the video electronics. If there is a difference between reference and actual frequency of the mirror, the maximum mirror deflection is not reached. The intensity of the effect is dependent on the production quality of the mirrors (manufacturing-induced reference frequency deviation). Image jitter effects can also occur as a result of a change in the ambient conditions (e.g. temperature, air pressure, atmospheric humidity, etc.) (environment-induced reference frequency deviation).
It is known that the drive frequency of the fast axis of the beam deflection system correctively adjusts its mechanical resonant frequency and is used as a time base. The corrective adjustment of the frequency is necessary in order to keep the geometric size of the projected image constant. The time base causes a trigger signal to be sent to the data processing unit (DPU), which has a fixed system clock, e.g. after each reversal of direction of the mirror's rotational movement in order to start the intensity modulation of the laser beams along a row.
This method has the disadvantage that if the frequency of the time base is changed (e.g. as a result of a thermal drift of the mechanical resonance), image information at the end of the row will be truncated or rows will be displayed compressed.
This leads to a reduction in the quality of the projected image.